Competing Energy Transfer Pathways in a Five-Chromophore

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Competing Energy Transfer Pathways in a Five-Chromophore Perylene Array Vineeth Benyamin Yasarapudi, Laszlo Frazer, James Edric Alan Webb, Joseph Keith Gallaher, Alexander Macmillan, Alexander Falber, Pall Thordarson, and Timothy W. Schmidt J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b01084 • Publication Date (Web): 24 Feb 2018 Downloaded from http://pubs.acs.org on February 24, 2018

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The Journal of Physical Chemistry

Competing Energy Transfer Pathways in a Five-Chromophore Perylene Array Vineeth B. Yasarapudi,

Gallaher,

†

†

Laszlo Frazer,

¶

Alexander Macmillan,

†,§

James E. A. Webb,

‡

Alexander Falber,

‡

Joseph K.

Pall Thordarson,

‡

and

∗,†

Timothy W. Schmidt

†ARC

Centre of Excellence in Exciton Science, School of Chemistry, UNSW Sydney, NSW 2052, Australia. ‡Australian Centre for Nanomedicine and the ARC Centre of Excellence in Bio-Nano Science & Technology, School of Chemistry, UNSW Sydney, NSW 2052, Australia. ¶Mark Wainwright Analytical Centre, UNSW Sydney, NSW 2052, Australia. §ARC Centre of Excellence in Exciton Science, School of Chemistry, Monash University, Clayton, VIC 3800, Australia. E-mail: [email protected]

1

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Abstract A perylene (donor-dimer)-acceptor-(donor-dimer) pentamer array is synthesized to investigate the competition between excimer formation and Förster resonance energy transfer. Using time-resolved uorescence, we show that, upon excitation, the isolated perylene dimer forms an excimer with a time constant of 4.3 ns. However in the pentamer array, when either of two constituent dimers donate their energy to the acceptor uorophore, the excimer energy trap is eliminated. The pentamer macromolecule shows broad absorption and reduced self-absorption, at some cost to uorescence quantum yield.

Introduction Global solar energy reserves are on the order of by three orders of magnitude.

1

1016 W,

which dwarfs current consumption

Therefore light-driven devices can be cheap to operate. The

eectiveness of organic light- or electrically-driven devices designed to produce photocurrent, light, or chemical reactions depends on controlled generation and transport of excitons. The controlled transport of excitonic energy relies on the interactions between chromophores,

2

which may or not be covalently bonded. In organic solar cells,

3

the photogenerated exciton must diuse to the heterojunction. In

organic light-emitting diodes the electrically generated exciton ideally migrates to a highly luminescent dopant.

4,5

Dendrimers can be useful for directing excitons to an active site

which performs re-emission,

69

charge separation,

10

or a chemical reaction.

11

Photosystem

II is an example where multiple chlorophylls sensitize reaction centers. In single molecule devices, absorption can be strengthened by covalently bonding more chromophores to the active component.

12

When attaching a large number of chromophores to an active site, it is necessary that the chromophores are close. If two similar-energy chromophores are closely situated, even H-aggregated, and one is optically excited, they can form an excimer.

2


2,1320

The excimer is

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often a detrimental energy trap, and usually exhibits a cancellation of radiative transition moments which could hinder subsequent Förster resonance energy transfer (FRET).

21

The

competition between excimer formation and FRET is thus of relevance to a range of excitonic devices.

O C O

O N

C O O

O C8H9 O

N

O

O O

O O

O

N O C8H9

Scheme 1: Structure of the monoimide diester donor (top) and tetraphenoxy diimide acceptor (bottom) in monomer form.

In this paper, we investigate the competition between excimer formation and FRET in a ve-chromophore perylene array consisting of an acceptor and two donor dimers. acceptor is a perylene diimide bay-functionalized with four phenoxyl groups.

The

The donor

chromophore is a perylene monoimide diester, shown in Scheme 1. These are ligated to form a dimer which readily forms an excimer.

22

The dimer is shown in Scheme 2.

Rather than attempting to separate the two parts of the dimer, we are interested in the competition between excimer formation and FRET, and have implemented an architecture for rapid energy transfer. The perylene pentamer resulting from two design principles (increasing the number of chromophores and rapid energy transfer) is shown in Scheme 2. The acceptor is functionalized so that its absorption spectrum broadens the region of the spectrum covered by the pentamer. As a side eect of the energy transfer design, the self-absorption of the array is reduced. If the uorescence quantum yield of this system can be increased, it may be suitable for

3



O O C

O N

O

O

O

O

O O C

O

O

N

C O O

O

O O

O O

O

N

O O C O

O

O N

N O

O

O C O

O C O

O

O

O O C

C O O

O

O N

N O

O

O C O

O C O

O

O

O C O

Page 4 of 23

O

O O

N C O O

O O

O

Scheme 2: Structure of the donating dimer (top) and pentamer (bottom).

luminescent solar concentration, a technology where uorescence drives solar energy into a large and cheap waveguide, ultimately sending it to a small, relatively high cost solar cell. In the absence of perfect uorescence quantum yield, self-absorption in a device which contains concentrated uorophores, such as a luminescent solar concentrator, can make the device inecient.

n

23,24

If the uorophore's uorescence quantum yield is

absorption-and-uorescence cycles, the yield is reduced to

yn.

Experimental Synthesis The synthesis methods were developed from the work of June et al.

4

then after

As the number of cycles

increases, energy is quickly lost.


y,

25

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Dimer Dimethylformamide (5 mL) was added to di(bromopropyl) isophthalate (40 mg, 98 µmol) and 3,4-dibutylcarboxylate-9,10-imidoperylene (170 mg, 328 µmol) with caesium carbonate

◦ (70 mg, 210 µmol). This was placed in an oil bath at 80 C with stirring for 3 hours. The reaction was then worked up by addition of aqueous hydrochloric acid (50 mL, 2 M) then ltered and air dried.

The resultant material was then puried by preparative thin layer

chromatography (1% methanol in dichloromethane) to yield the dimer as a red solid (89 mg, 69 µmol, 70%).

+

+

+

+

HR-ESI: [(C78 H68 N2 O16 Na) ]=[(M Na) ], calc. 1311.4461, found 1311.4463.

1

H NMR

(300 MHz, CDCl3 ): 9.08 (1H, t, 1.6, HAr -isophthalic), 8.36 (2H, dd, 7.8, 1.6, HAr -isophthalic), 8.11 (4H, d, 8.1, HAr -perylene), 7.86 (4H, d, 8.0, HAr -perylene), 7.73 (4H, d, 8.0, HAr perylene), 7.69 (4H, d, 8.1, HAr -perylene), 7.62 (1H, t, 7.8, HAr -isophthalic), 4.57 (4H, m, propyl X−[CH2 ]−CH2 ), 4.51 (4H, m, propyl X−[CH2 ]−CH2 ), 4.40 (8H, t, 6.9, butyl O−[CH2 ]−), 2.39 (4H, br m), 1.87 (8H, q, 7.2, butyl OCH2 −[CH2 ]−), 1.56 (8H, sx, 7.5, butyl CH2 −[CH2 ]−CH3 ), 1.05 (12H, t, 7.4, butyl CH2 −[CH3 ]).

Pentamer Tetraphenoxy

di(di(bromopropyl)imidoisophthalate)perylene:

p

Tetra( -tBu-phenoxy)

di(imidoisophthalic acid) perylene (1.25 g, 0.95 mmol), 1,3-dibromopropane (60 mL) and potassium carbonate (5.0 g) were combined along with water (30 mL) and placed into an

◦ oil bath at 110 C for 3 hours. Chloroform was then added to the reaction and the organic phase separated and passed through a celite plug.

The solvent was then removed and

the residue puried on silica with a 300:100:1 dichloromethane:hexane:isopropanol solvent system.

p

This produced the tetra( -tBu-phenoxy) di(di(bromopropyl) imidoisphthalate)

perylene as a dark blue solid (786 mg, 0.43 mmol, 45%).

1

H NMR (300 MHz, CDCl3 ):

8.75 (2H, t, 1.5, HAr -isophthalic), 8.26 (4H, s, HAr -

perylene), 8.13 (4H, d, 1.5, HAr -isophthalic), 7.24 (8H, d, HAr -phenoxy), 6.86 (8H, d, 8.8,

5



Page 6 of 23

HAr -phenoxy), 4.49 (8H, t, 6.1, O−[CH2 ]−), 3.51 (8H, t, 6.5, Br−[CH2 ]−), 2.31 (8H, quin,

p

6.3, CH2 −[CH2 ]−CH2 ), 1.27 (36H, s, [ -tBu]-phenoxy).

p

Tetra( -tBu-phenoxy) di(di(bromopropyl) imidoisophthalate) perylene (100 mg, 55.6 µmol) was combined in a sealed vial with monoimide dibutylester perylene (310 mg, 0.59 mmol) in N-methylpyrolidinone (20 mL) along with potassium carbonate (140 mg) and tetrabutylam-

◦ monium bromide (60 mg). This was then placed into an oil bath at 120 C for 45 minutes. The reaction was precipitated by the addition of methanol (30 mL) and ltered, washing the precipitate with methanol (60 mL). The solid was then recrystallised from neat toluene

p

by addition of methanol to yield the tetra(propylimide dibutylester perylene) mono(tetra( tBu-phenoxy) di(imidoisophthalate) perylene) as a dark red solid (147 mg, 41 µmol, 74%).

+

+

+

+

HR-ESI: [(C220 H190 N6 O40 Na2 )2 ]=[(M Na2 )2 ], calc. 1801.6435, found 1801.6432.

1

H

NMR (300 MHz, CDCl3 ): 9.14 (2H, t, 1.5, HAr -isophthalic), 8.31 (4H, s, HAr -core perylene), 8.29 (4H, d, 1.4, HAr -isophthalic), 8.15 (8H, d, 8.0, HAr -appending perylene), 7.88 (8H, d, 8.0, HAr -appending perylene), 7.77 (8H, d, 8.0, HAr -appending perylene), 7.74 (8H, d, 8.0, HAr -appending perylene), 7.27 (8H, d, 9.0, HAr -core perylene phenoxy), 6.89 (8H, d, 9.0, HAr core perylene phenoxy), 4.54 (16H, br m, O−[CH2 ]−CH2 −[CH2 ]−N), 4.40 (16H, t, 6.9, butyl O−[CH2 ]−), 2.38 (8H, br m, OCH2 −[CH2 ]−CH2 N), 1.87 (8H, m, butyl OCH2 −[CH2 ]−), 1.29 (36H, s, phenoxy-[tBu]), 1.05 (24H, t, 7.4 Hz, butyl

−[CH3 ]).

Steady-state spectroscopy Absorption measurements were carried out using a Varian Cary 50 Bio spectrophotometer. Emission and excitation-emission spectra were recorded using a Cary Eclipse uorimeter congured with 5 nm resolution. Fluorescence quantum yields were measured relative to a reference. prepared in ethanol.

26

The reference was

The donor was referenced to 2,3,6,7-Tetrahydro-9-(triuoromethyl)-

1H,5H,11H-(1)benzopyrano(6,7,8-

i,j )quinolizin-11-one (Coumarin 540a). The acceptor, pre-

viously reported trimer D-A1-D ,

12

and pentamer were referenced to 4-(dicyanomethylene)-

6


Page 7 of 23

2-methyl-6-(4-dimethylaminostyryl)-4H-pyran (trade name DCM dye).

The results at the

excitation wavelengths used for quantum yield measurements are indicated in Table 1.

Time-resolved uorescence Time-resolved spectra were recorded using a Horiba Fluoromax time-correlated single photon counting instrument. The samples were excited using a 468 nm pulsed laser diode so that the light is primarily absorbed by the donor. The instrument response time is less than 1 ns. The spectral resolution is 10 nm or better. The dimer time-resolved uorescence is shown in Fig.

1.

The pentamer time-resolved

uorescence is shown in Fig. 2.

10000

Time (ns)

100

1000

80 100

60 40

10

20 0

Brightness (counts)

120

1 800

750

700

650

600

550

500

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Wavelength (nm) Figure 1: Time-resolved uorescence spectrum of the dimer at a concentration of 1 µM.

Decomposition of time-resolved uorescence Singular value decomposition indicated that the dimer time-resolved uorescence included two signicant linearly independent components. The pentamer time-resolved uorescence

7



10000

Time (ns)

100

1000

80 100

60 40

10

20 0

Brightness (counts)

120

1 800

750

700

650

600

550

500

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 23

Wavelength (nm) Figure 2: Time-resolved uorescence spectrum of the pentamer, showing dynamics with only one signicant linearly independent component.

had one component, indicating that decomposition was not required.

The dimer results

were decomposed using the Multivariate Curve Resolution - Alternating Least Squares algorithm.

27

The two-component decomposition was initialized using evolving factor analysis.

As uorescence is strictly non-negative, the model was constrained to be non-negative.

Results and Discussion Steady state spectra towards energy transfer As in most uorophores, the absorption and uorescence spectra of perylenes overlap in the region of the spectrum near the rst singlet excited state. Overlap can cause self-absorption of uorescence. In Fig. 3, the overlap in the monoimide diester donor occurs in the range 500-530 nm. In the tetraphenoxy diimide acceptor, it occurs in the range 560-600 nm. Fig. 4 shows that the self-absorption of the pentamer is suppressed relative to the selfabsorption of the donor alone, because the emission spectrum is oset towards longer wave-

8


Page 9 of 23

-1

-1

Molar Absorptivity (M cm )

80000 Donor Acceptor

70000

1 0.8

60000 50000

0.6

40000 0.4

30000 20000

0.2 10000

Emission (Relative to Peak)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0 0 350 400 450 500 550 600 650 700 750 800 Wavelength (nm) Figure 3: Steady-state absorption (solid lines) and emission (dashed lines) spectra of the perylene donor and acceptor monomers. The donor is excited at 500 nm. The acceptor is excited at 530 nm.

lengths by about 80 nm. Meanwhile, the pentamer absorption peak is oset towards shorter wavelengths by about 100 nm relative to the acceptor. In Fig. 4, we nd that the self-absorption of the dimer is even less than the self-absorption of the pentamer, even though the dimer lacks an acceptor to sensitize. This is caused by the dimer's tendency to form an excimer. Table 1 shows the self-absorption ratio,

S,

for each

uorophore, which is the absorption at the absorption peak divided by the absorption at the emission peak.

28

Table 1: Quantum Yields and Self-Absorption Ratios (S ). Molecule

Excitation wavelength

Donor

1

460

0.81(4)

3.2

Acceptor

1

520

0.78(2)

4.8 2

Dimer Trimer D-A1-D Pentamer

12

Quantum yield

S

Fluorophores

∼ 3 × 10

2

-

low

3

495

0.62(2)

9

5

480

0.54(3)

6.0

9



-1

-1

Molar Absorptivity (M cm )

80000 Dimer Pentamer

70000

1 0.8

60000 50000

0.6

40000 0.4

30000 20000

0.2 10000

Emission (Relative to Peak)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 23

0 0 350 400 450 500 550 600 650 700 750 800 Wavelength (nm) Figure 4: Steady-state absorption (solid lines) and emission (dashed lines) spectra of the perylene dimer and pentamer. Both molecules are excited at 450 nm, so that the acceptor is not excited. The contribution of donor monomer or dimer emission to the pentamer emission spectrum is negligible, indicating ecient energy transfer to the pentamer's acceptor. Owing to small amounts of pentamer material available, accurate concentrations could not be obtained, and thus the measured absorptivity is likely inaccurate.

10


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Excimer formation in the donor dimer The dimer spectra are shifted relative to the donor monomer. The shift in the dimer absorption spectrum towards shorter wavelengths in Fig. 4 (dark blue) as compared to Fig. 3 (green) indicate H-aggregation in the dimer. To investigate the dynamics giving rise to the substantial shift of the dimer photoluminescence spectrum about 70 nm towards longer wavelengths, we performed time-resolved uorescence spectroscopy on each uorophore.

Fig.

5 shows that the donor, acceptor,

and pentamer all have similar uorescence kinetics at 600 nm.

However, the dimer uo-

rescence decays much more slowly. The reduced uorescence decay rate is symptomatic of H-aggregation and subsequent excimer formation in the dimer. In an H-aggregate, the radiative transition moments of the monomers partially cancel. This leads to a slower rate of spontaneous emission and often leads to a lower uorescence quantum yield if the rate of the slowed uorescence fails to exceed non-radiative decay rates. Aggregation of the dimer can occur in the ground state to form an H-aggregate, or in the excited state where it forms an excimer from free monomers, one having been excited. To isolate these two kinds of aggregation, we analyzed the uorescence decay at dierent wavelengths. Fig. 6 shows that there are actually two rate constants present in the dimer uorescence decay.

At shorter wavelengths, the faster decay rate predominates, while at

longer wavelengths the slower decay rate is most signicant. Singular value decomposition indicates that there are only two signicant components to the data. We used the multivariate curve resolution alternating least-squares (MCR-ALS) algorithm

27

to determine the uorescence spectra of the two components. In Fig. 7, the rst

component's spectrum shows two peaks. The peak near 520 nm corresponds to the donor monomer emission.

It is slightly shifted to longer wavelengths owing to weak excitonic

coupling to the second donor molecule in the dimer.

The second peak, near 570 nm, is

emission from dimers which are aggregated before they are excited. While these aggregated molecules presumably have a reduced uorescence rate, their total decay rate is high because

11



1 Brightness (Relative to Peak)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 23

Donor Acceptor Dimer Pentamer

0.1

0.01

0.001 0

20

40 60 Time (ns)

80

100

Figure 5: Time-resolved uorescence of the donor, acceptor, dimer, and pentamer at 600 nm emission. The perylenes all have similar uorescence decay rates, except the dimer (donor: 1.80(2) × 108 s−1 , acceptor: 1.40(1) × 108 s−1 , dimer: 4.434(8) × 107 s−1 , pentamer: 1.57(1) × 108 s−1 ). The dimer's uorescence lifetime is extended by H-aggregation.

12


Page 13 of 23

1 Brightness (Relative to Peak)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(nm) 510 530 550 570 590

0.1

0.01

0.001 0

20

40 60 Time (ns)

80

100

Figure 6: Fluorescence dynamics of the dimer at various emission wavelengths, showing the free singlet and excimer uorescence decay rates. For emission at 510 nm, the free singlet dominates. For emission at 590 nm and greater (not shown), the excimer emission dominates. The dimer concentration is 1 µM.

they form excimers promptly. The second component had only one peak, near 580 nm. We assign this emission to the excimer state of the dimer.

The bathochromic shift of each

component is smaller than the shift previously reported for a perylene dimer which was connected at both ends.

29

The temporal proles calculated using MCR-ALS are shown in Fig. 7. The residuals, which are negligible, are displayed in Fig. S1. The results indicate that the measured rise time of the rst component is similar to the instrument response time and can only be interpreted as a upper bound. The second component has a longer rise time. The decay rate of the rst component is equal to the rise rate of the second component, which is consistent with interconversion. This kinetic behavior is further evidence that the second component is an excimer, which forms only after the dimer is excited. Similar results have been obtained for pyrene-labeled dendrimers.

20

We modeled the two components simultaneously and concluded

that the excimer formation rate is

2.323(6) × 108 s−1 .

13

The excimer decay rate is


4.434(8) ×

1

Page 14 of 23

Component 1 Component 2

0.8 0.6 0.4 0.2 0 500

550

600

650

700

750

800

Wavelength (nm) Component Brightness (Relative to Peak)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Component Brightness (Relative to Peak)


100 10-1 10-2 10-3

0

5

Figure 7: Decomposition of Fig.

10

15 20 Time (ns)

25

30

35

40

1, the time-resolved uorescence of the dimer, showing

conversion of the initial excited state into an excimer. The initial excited state (Component 1) includes both monomer-like and H-aggregate-like emission. The excimer (Component 2) exhibits stronger excitonic coupling than Component 1. The solid curves are our model of the excimer kinetics.

14


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107 s−1 . The excimer formation rate constant includes a contribution from radiative decay of the rst MCR-ALS generated component. Based on the relative brightnesses of the components and the assumption that additional aggregation cannot increase quantum yield, we conclude that the excimer formation process dominates over the uorescence. This suggests the preexisting aggregation eects apparent in the rst component reduce its uorescence rate relative to the monomeric donor. The resulting (unmeasurable) radiative decay rate is less than the excimer formation rate.

Energy transfer from the donor to the acceptor We attempted photoluminescence upconversion measurements, but did not detect any luminescence kinetics on the picosecond scale, within the (∼

150 fs)

time-resolution of the

instrument. As such, in order to demonstrate successful energy transfer from the donor to the acceptor within the pentamer, we constructed models of the excitation-emission spectra of the pentamer. The models were built using the steady-state absorption and emission spectra of the dimer and the acceptor. In the model without energy transfer, each uorophore's absorption spectrum was assigned to its own emission spectrum.

In the model with perfect energy

transfer, the absorption spectrum of the dimer and the acceptor were both assigned to the emission spectrum of the acceptor. In Fig. 8, the model without energy transfer is dominated by broadband excimer emission. The model with perfect energy transfer shows the sharper emission peak of the acceptor, but with the combined excitation spectrum of the dimer and the acceptor. Our measurement of the pentamer excitation-emission spectrum is similar to the model with perfect energy transfer, and shows no evidence of excimer emission. conclude that energy was successfully routed from the donor to the acceptor.

15


We


Emission Wavelength (nm)

700 650 600 550

1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0

1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0

Brightness (arbitrary)

750


400 450 500 550 600 Excitation Wavelength (nm)


750 700 650 600 550 400 450 500 550 600 Excitation Wavelength (nm)


750

80 70

700

60 50

650

40 30

600

20


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 23

10 550

0 400 450 500 550 600 Excitation Wavelength (nm)

Figure 8: Excitation-emission spectra. Top: Model without energy transfer. Middle: Model with perfect energy transfer.

Bottom:

Measurement of the pentamer.

The pentamer

excitation-emission spectrum is consistent with perfect energy transfer. No donor emission is observed, and the excitation spectrum of the acceptor is consistent with ecient energy transfer from the donor. The white areas indicate that data is removed because the emission wavelength is similar to or shorter than the excitation wavelength.

16


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Inhibition of excimer formation Excimer formation can function as an energy trap. In the pentamer, we nd no evidence of excimers in the emission spectra, regardless of the excitation wavelength. The pentamer uorescence decay strongly resembles the uorescence decay of a perylene monomer. Any excimer-like kinetics are constrained to contribute less than 1% to the total uorescence kinetics. Singular value decomposition of the spectrally resolved uorescence decay did not reveal a signicant excimer component in the pentamer, unlike the dimer. We conclude that the reduced uorescence quantum yield of the pentamer (Table 1) is caused by an increased nonradiative decay rate from additional vibronic coupling, not an excimer-induced reduction in the uorescence decay rate. We designed the pentamer to exhibit FRET,

12

where the donor emission overlaps the

acceptor absorption. Overall, the evidence shows that the rapid rate of energy transfer from the donor to the acceptor exceeds that of excimer formation, even in pre-aggregated dimers, eliminating energy trapping. Previous measurements of perylene donor-acceptor macromolecules fer rates of about

1012 s−1 .

12

showed energy trans-

We found that the energy transfer rate for the pentamer was

within the response time of the time correlated single photon counting instrument, and the dynamics could not be resolved by uorescence upconversion. Therefore we conclude that the excimer suppression is achieved with an energy transfer rate larger than

1012 s−1 .

Conclusion Upon absorption of light, the covalent perylene dimer formed an excimer, which reduced self-absorption losses. But, excimer formation suppresses the radiative decay rate, and thus is not a viable strategy for luminescence solar concentration. By covalently linking the dimer with a perylene acceptor, we successfully traded a lit-

17



2.5 eV Donor > 10 1 2 s −1

8

10

−

s

1

2.2 eV Excimer 2.1 eV Acceptor 2 × 108 s−1

4 × 107 s−1

Excitation

2×

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Ground State

Figure 9: Energy levels and energy conversion rates in the pentamer. Energy transfer to the acceptor and acceptor uorescence are the dominant processes. Not to scale.

tle self-absorption for the elimination of excimer formation. The competition between the dimer's excimer formation and energy transfer rates is summarized in Fig. 9. The

tert -butyl phenoxy groups in the bay of the acceptor shift the acceptor energy level

to drive ecient energy transfer while inhibiting excimer formation between the donor and acceptor units. We have shown that energy transfer is still ecient in a ve-chromophore system.

Further increases in number of donor units and increased uorescence quantum

yields will enhance the usefulness of these arrays as light convertors. Our design successfully inhibits excimer formation and self-absorption.

Supporting Information Available The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.XXXXXX. Residuals of the decomposition in Fig. 7 of the main text.

Acknowledgement This work was supported by the Australian Research Council Centre of Excellence in Exciton Science (CE170100026). T.W.S. acknowledges the Australian Research Council for a Future Fellowship (FT130100177) and P.T. acknowledges the Australian Research Council for a

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Linkage Grant (LP130100774). This research is supported by Flurosol Industries Pty Ltd.

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TOC Graphic

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Competing Energy Transfer Pathways in a Five-Chromophore

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